Nitrobenzene is an important intermediate of the chemical industry, which is needed in particular for the production of aniline and hence also for the production of di- and polyisocyanates of the diphenylmethane series and of the polyurethanes based thereon.
The nitration of benzene with nitric acid to give a crude nitrobenzene has already been the subject-matter of numerous publications and patent applications. The methods in common usage today correspond substantially to the concept of the adiabatic nitration of benzene with a mixture of sulfuric and nitric acid (known as a mixed acid). Such a method was first claimed in U.S. Pat. No. 2,256,999 and is described in its modern embodiments in EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247 B2, for example. The methods involving adiabatic reaction control are characterised in particular by the fact that no technical measures are taken to introduce heat into or to dissipate heat from the reaction mixture.
Isothermal methods for the nitration of benzene with mixed acid are also described, as described for example in EP 0 156 199 B1.
Methods for the nitration of benzene are also known that manage without the use of sulfuric acid. These are described for example in U.S. Pat. No. 2,739,174 or U.S. Pat. No. 3,780,116.
Gas-phase methods for the nitration of benzene with nitric acid or nitrogen oxides are also possible in principle, but the yields obtained with them are still low (EP 0 078 247 B1, EP 0 552 130 B1).
Common to all of these methods is the fact that a crude nitrobenzene is formed first as the reaction product, which contains nitric acid and, if nitrated with mixed acid, sulfuric acid as impurities and dinitrobenzene and nitrated oxidation products of benzene, in particular nitrated phenols (nitrophenols), as organic impurities. It also contains organic compounds formed from compounds contained as impurities in the benzene that was used (WO 2008/148608 A1). The crude nitrobenzene moreover also contains metal salts, which can be present in dissolved form in the acid residues or in the crude nitrobenzene (DE 10 2007 059 513 A1).
Countless studies in the past have aimed at improving the quality of the crude nitrobenzene and hence increasing the yield of benzene and nitric acid. Thanks to these developments, the modern adiabatic liquid-phase methods have advanced to such an extent that they all succeed in producing a crude nitrobenzene having a low content of byproducts, i.e. on average only 100 ppm to 300 ppm of dinitrobenzene and 1500 ppm to 2500 ppm of nitrophenols, wherein picric acid can accept a proportion of 10% to 50% of the nitrophenols.
The crude nitrobenzene still contains as impurities water, benzene, nitrophenols and dinitrobenzene and, if mixed acid was used for nitration, sulfuric acid. These impurities are undesirable, as they can have a negative influence on downstream processes in which nitrobenzene is used, such as for example the production of aniline. Suitable processing methods including washing and distillation stages are described for example in U.S. Pat. No. 6,288,289 B1, EP 1 593 654 A1, EP 1 816 117 B1 and WO2011/021057 A1.
EP 1 816 117 B1 describes the processing of crude nitrobenzene in an acid wash, an alkaline wash with aqueous sodium hydroxide solution, a neutral wash and a final purification by distillation. Bases other than sodium hydroxide solution, such as for example aqueous sodium carbonate solution or aqueous ammonia solution (WO 2011/082 977 A1) or potassium hydroxide or ammonia (DE 60 113 579 T2), can of course also be used in principle.
Another embodiment of the processing of crude nitrobenzene is described in WO2011/021057 A1, in which the problem of salts in the washing process is addressed in detail. The crude nitrobenzene is washed with water in the first step, then subjected to an alkaline wash with sodium hydroxide solution and finally washed with acid before being subjected to steam stripping to remove water and excess benzene. The preferred acid is nitric acid, which already occurs in the nitration process and is volatile, so it can be removed from the product at the steam stripping stage. This embodiment has the disadvantage that a certain amount of sodium hydroxide solution and sodium nitrophenolates is rinsed out of the alkaline wash into the next washing stage, in this case an acid wash. The reaction of sodium nitrophenolates with nitric acid releases nitrophenols, which in turn find their way into the crude nitrobenzene as an impurity. One aim of the acid wash, that of completely removing these compounds from the product, is thus no longer achieved.
The purified nitrobenzene (pure nitrobenzene) is predominantly used in the production of aniline, which in turn is predominantly carried out today by the catalytic hydrogenation of nitrobenzene in the gas phase with hydrogen. To convert it into the gas phase, nitrobenzene can either be evaporated (EP 0 696 574 B1, paragraph [0024]) or sprayed into a hot gas stream, preferably into a hydrogen stream (DE-OS-1 809 711, DE 10 2006 035 203 A1, paragraph [0053]). The use of evaporation is regarded as advantageous, as it is said to result in far fewer deposits in the reactor and in the supply lines (EP 0 696 574 B1, paragraph [0024]). Metal compounds, salts and high-boiling solvents in the nitrobenzene tend instead to remain in the evaporator and do not find their way into the reaction system. The complexity of the apparatus required for evaporating large quantities of nitrobenzene is considerable, however, so spraying is used in many places. Here nitrobenzene is sprayed into the circulating gas stream of the hydrogenation plant, such that metal compounds, salts and high-boiling solvents in the nitrobenzene find their way into the reactor. This results in undesirable deposits, such that the reactor cleaning intervals are reduced and the catalyst can deactivate prematurely. Salts that poison the catalyst are especially damaging, as even very small amounts are sufficient to bring about a deactivation.
There was therefore a need for a method for producing nitrobenzene that provides nitrobenzene in a quality such that in an aniline gas-phase process it does not lead to a loss of operating stability and/or catalyst activity, even if the spraying method is used to convert the nitrobenzene into the gas phase. Furthermore, the nitration process itself should be able to be performed with as few problems as possible (no plant malfunctions caused by emulsification in the neutral wash for instance).